NATO officially launched the process of revamping its Strategic Concept this month. As of now, the alliance’s next mission statement is “a blank sheet of paper,” in the words of outgoing Secretary General Jaap de Hoop Scheffer. But NATO’s recent pronouncements and current challenges offer plenty of guidance on how to fill the page.

First, NATO nations must invest more in defense, and they must remember that theirs is, above all, a military alliance. As the secretary general observed, military operations are “NATO’s core business.”

The U.S. spends about 4 percent of its GDP on defense, but only four other NATO members have mustered the will to meet the alliance’s standard of investing just 2 percent. NATO’s combat effectiveness is suffering as a consequence: Some NATO members have to hitch a ride with the U.S. Air Force or rent Soviet-era transports to deploy to Afghanistan; they “are not trained in counterinsurgency,” in the words of Defense Secretary Robert Gates; and they lack helicopters to move across Afghanistan.

This was on full display earlier this month, when Britain’s top military commander flew into Helmand province in a U.S. Blackhawk helicopter. Britain has just 30 choppers to support its 9,100-man contingent in Afghanistan -- and Britain is considered America’s nearest peer within NATO.

There will always be a capabilities gap between the U.S. and its NATO allies, but the alliance cannot devolve into a one-for-all public good.

Second, NATO needs to make its words matter. Only once in its history has NATO invoked Article 5, the alliance’s collective defense clause. That was on Sept. 12, 2001.

However, some NATO members don’t seem to take Article 5 seriously. If they did, Washington wouldn’t have to beg for more troops to support NATO’s Afghanistan mission, and the troops that are there wouldn’t have limits on where they can go.

Even the secretary general has noted that “the new Strategic Concept should . . . reassure our new allies that NATO takes its Article 5 collective defense commitment seriously.”

One way to do so is to abolish what NATO euphemistically calls “caveats,” which allow members to opt out of combat operations. As a blue-ribbon panel of former NATO commanders argues, caveats “prevent the operational commander from making adequate use of allocated forces.” Worse, they strike at the very heart of the alliance’s cohesiveness. After all, an ally that promises to help only when the guns are silent or where the scenery is serene is not much of an ally.

More significantly still, if NATO’s own don’t take Article 5 seriously, neither will its enemies.

On a related matter, NATO should not allow members without troops in the fight to dictate how others conduct it. “Only those nations that contribute . . . military forces in a military operation should have the right to a say in the process of the operation,” the former commanders wisely conclude. NATO simply cannot function as an effective military organization if its battle plans and missions are shaped by the lowest common denominator.

Third, NATO needs to be creative about expansion. While the 2009 NATO summit promised that “NATO’s door will remain open,” Russia’s lunge into Georgia has had a chilling effect on NATO expansion. That doesn’t mean NATO should stop growing, however.

Once a small club of nations clustered around the Atlantic Ocean, NATO now enfolds a wide swath of the northern hemisphere. NATO maintains what it calls “unique” partnerships with Australia, Japan, New Zealand, Singapore and South Korea. Perhaps these partnerships will need to grow into something more formal, as NATO suggested in 2008.

Finally, and most importantly, NATO needs to expect the unexpected. After all, when NATO’s last Strategic Concept was approved, no one envisioned that the alliance would be where it is today. Indeed, reading the 1999 Strategic Concept is like sifting through a time capsule. The document focuses on “ethnic and religious rivalries” in Europe, commits the alliance to “effective conflict prevention . . . and wide-ranging partnership, cooperation, and dialogue with other countries in the Euro-Atlantic area,” and outlines the essential roles of the U.N., EU and OSCE.

Strikingly lacking is anything about the Arctic, Africa, Afghanistan or al-Qaida. In fact, terrorism is something of an afterthought, mentioned alongside organized crime and sabotage.

Ten years later, the alliance includes 28 members, enfolds virtually all of Europe, and is waging war in Afghanistan. NATO has deployed forces to North America, fought piracy off the Horn of Africa, transported African Union peacekeepers, trained Iraqi soldiers, and assisted a member nation crippled by cyber-attacks.

Just as these missions were not on NATO’s radar a decade ago, the form and function of tomorrow’s NATO is impossible to predict.

Will the NATO of 2019 be an extension of Washington or of the EU? Will it be “an expeditionary alliance,” as former President George W. Bush envisioned, stabilizing the world’s trouble spots in order to protect NATO’s core? Will it help “combat fear and want wherever they exist,” as President Barack Obama has suggested?

Will it become a global gendarme or a mini-U.N.? Will it need to carry out military operations in space or cyberspace, airstrikes in Iran or Lebanon, humanitarian interventions in North Korea or Nigeria, peacekeeping missions in Palestine, defensive maneuvers in the Arctic, and airlifts into Tbilisi, Georgia -- or, for that matter, Atlanta, Georgia? Or will it retrench, retreat and retract?

The new Strategic Concept may not answer those questions. But it needs to at least contemplate them if it is to help tomorrow’s NATO answer them.